My PyQt plot's Y axes are upside down (even the text)?

Question

Today is the first day I have tried using PyQtGraph. I really like it so far except I can't seem to fully comprehend how things work..

I am trying to place two FFT plot widgets into the same window. After much trial and error I found what I thought was the proper way to do it. However now I have two plots which show the correct information but everything on the Y axis is inverted.

Also it seems zooming and panning are not correct either (the whole plot moves, not just the data within it).

This image shows the two real-time audio fft plots both within a single GraphicsWindow. On the left I use addPlot with addItem and on the right I use addViewBox with addItem.

To be thorough I have tried using item.invertY(True) and item.scale(1,-1). In both cases it will invert the Y axis data but not the text or axes, nor does it address the panning/zooming issues..

This Python script is everything I was able to write.

It was based off of this file: pyqtgraph live running spectrogram from microphone

import numpy as np
import pyqtgraph as pg
import pyaudio
from PyQt4 import QtCore, QtGui

FS = 44100 #Hz
CHUNKSZ = 1024 #samples

class MicrophoneRecorder():
    def __init__(self, signal):
        self.signal = signal
        self.p = pyaudio.PyAudio()
        self.stream = self.p.open(format=pyaudio.paInt16,
                            channels=1,
                            rate=FS,
                            input=True,
                            frames_per_buffer=CHUNKSZ)

    def read(self):
        data = self.stream.read(CHUNKSZ)
        y = np.fromstring(data, 'int16')
        self.signal.emit(y)

    def close(self):
        self.stream.stop_stream()
        self.stream.close()
        self.p.terminate()



class SpectrogramWidget2(pg.PlotWidget):

    read_collected = QtCore.pyqtSignal(np.ndarray)
    def __init__(self):
        super(SpectrogramWidget2, self).__init__()

        self.img = pg.ImageItem()
        self.addItem(self.img)

        self.img_array = np.zeros((1000, CHUNKSZ/2+1))

        # bipolar colormap
        pos = np.array([0., 0.5, 1.])
        color = np.array([[0,0,0,255], [0,255,0,255], [255,0,0,255]], dtype=np.ubyte)
        cmap = pg.ColorMap(pos, color)
        pg.colormap
        lut = cmap.getLookupTable(0.0, 1.0, 256)

        # set colormap
        self.img.setLookupTable(lut)
        self.img.setLevels([0,100])

        # setup the correct scaling for y-axis
        freq = np.arange((CHUNKSZ/2)+1)/(float(CHUNKSZ)/FS)
        yscale = 1.0/(self.img_array.shape[1]/freq[-1])

        self.img.scale((1./FS)*CHUNKSZ, yscale)

        self.setLabel('left', 'Frequency', units='Hz')

        # prepare window for later use
        self.win = np.hanning(CHUNKSZ)
        #self.show()

    def update(self, chunk):
        # normalized, windowed frequencies in data chunk
        spec = np.fft.rfft(chunk*self.win) / CHUNKSZ
        # get magnitude 
        psd = abs(spec)
        # convert to dB scaleaxis
        psd = 20 * np.log10(psd)

        # roll down one and replace leading edge with new data
        self.img_array = np.roll(self.img_array, -1, 0)
        self.img_array[-1:] = psd

        self.img.setImage(self.img_array, autoLevels=False)

class SpectrogramWidget(pg.PlotWidget):

    read_collected = QtCore.pyqtSignal(np.ndarray)
    def __init__(self):
        super(SpectrogramWidget, self).__init__()

        self.img = pg.ImageItem()
        self.addItem(self.img)

        self.img_array = np.zeros((1000, CHUNKSZ/2+1))

        # bipolar colormap
        pos = np.array([0., 0.5, 1.])
        color = np.array([[0,0,0,255], [0,255,0,255], [255,0,0,255]], dtype=np.ubyte)
        cmap = pg.ColorMap(pos, color)
        pg.colormap
        lut = cmap.getLookupTable(0.0, 1.0, 256)

        # set colormap
        self.img.setLookupTable(lut)
        self.img.setLevels([0,100])

        # setup the correct scaling for y-axis
        freq = np.arange((CHUNKSZ/2)+1)/(float(CHUNKSZ)/FS)
        yscale = 1.0/(self.img_array.shape[1]/freq[-1])

        self.img.scale((1./FS)*CHUNKSZ, yscale)

        self.setLabel('left', 'Frequency', units='Hz')

        # prepare window for later use
        self.win = np.hanning(CHUNKSZ)
        #self.show()

    def update(self, chunk):
        # normalized, windowed frequencies in data chunk
        spec = np.fft.rfft(chunk*self.win) / CHUNKSZ
        # get magnitude 
        psd = abs(spec)
        # convert to dB scaleaxis
        psd = 20 * np.log10(psd)

        # roll down one and replace leading edge with new data
        self.img_array = np.roll(self.img_array, -1, 0)
        self.img_array[-1:] = psd

        self.img.setImage(self.img_array, autoLevels=False)

if __name__ == '__main__':
    app = QtGui.QApplication([])


    win = pg.GraphicsWindow(title="Basic plotting examples")
    #win.resize(1000,600)


    w = SpectrogramWidget()
    w.read_collected.connect(w.update)

    spectrum1 = win.addPlot(title="Spectrum 1")#win.addViewBox()

    item = w.getPlotItem()

    spectrum1.addItem(item)


    w2 = SpectrogramWidget2()
    w2.read_collected.connect(w2.update)

    spectrum2 = win.addViewBox()
    spectrum2.addItem(w2.getPlotItem())

    mic = MicrophoneRecorder(w.read_collected)
    mic2 = MicrophoneRecorder(w2.read_collected)

    # time (seconds) between reads
    interval = FS/CHUNKSZ

    t = QtCore.QTimer()
    t.timeout.connect(mic.read)
    t.start((1000/interval) ) #QTimer takes ms

    t2 = QtCore.QTimer()
    t2.timeout.connect(mic2.read)
    t2.start((1000/interval) ) #QTimer takes ms

    app.exec_()
    mic.close()

Thank you for any help!

Answer 1

I have no idea why doing this causes things to be mirrored, but the issue is related to using the plotItem from a plot in another plot (I think that's what you're doing?)

Anyway, PlotWidgets shouldn't be used like that. They are just normal Qt Widgets, so add them to a Qt layout like you would with any other Qt Widget.

if __name__ == '__main__':
    app = QtGui.QApplication([])

    win = QtGui.QMainWindow()
    widget = QtGui.QWidget()
    win.setCentralWidget(widget)
    layout = QtGui.QHBoxLayout(widget)
    win.show()

    w = SpectrogramWidget()
    w.read_collected.connect(w.update)
    layout.addWidget(w)

    w2 = SpectrogramWidget2()
    w2.read_collected.connect(w2.update)
    layout.addWidget(w2)

    # .... etc

P.S. Is there a reason you have two identical classes with a different name? You could just instantiate multiple copies of the same class. E.g.

w = SpectrogramWidget()
w2 = SpectrogramWidget()